Dark Side of the Disc

The June issue of Stereophile, which hits newsstands this week, spills some ink on the 30th-anniversary reissue of Pink Floyd's Dark Side of the Moon as a two-layer Super Audio CD (Capitol CDP 582136 2). Jon Iverson nominated the disc as June's "Recording of the Month," while I mentioned it in my "As We See It" column. This "fully loaded" SACD includes both multichannel and two-channel mixes encoded with the DSD system on a high-rez SACD layer and a two-channel "Red Book" transfer (16-bit word length, 44.1kHz sampling) on its CD layer.

Jon wrote at length about the audible differences between the SACD-layer mixes and the CD-layer transfer, while I wrote about the measured differences. As the single-page format of the paper magazine's "As We See It" and "Recording of the Month" columns doesn't allow space for illustrations, this Web article should be read in conjunction with both. (Jon also wrote about the Crest-pressed SACD of DSotM developing radial cracks at its center in an online article.)

Both two-channel mixes of DSotM were claimed by EMI to have been transferred straight from the original analog master tape. It came as a surprise to both Jon and me, therefore, to hear relatively large differences between the SACD and CD versions, not the least of which was that the latter was louder.

In addition, since I recorded an album at Abbey Road Studio at the same time that the Floyd were there making DSotM, I always thought the album did an excellent job of preserving the characteristic sound of the studio with which I had become so familiar. Yet when I first listened to the CD layer of the reissue, it didn't sound like Abbey Road at all. The sonic subtleties that identify the recording venue and its unique reverb chamber had been eliminated or smoothed over. They were there on the SACD, so some investigation was called for.

I used the analytical capabilities of Syntrillium's Cool Edit Pro digital audio workstation PC program to look at the differences. I first ripped the CD-layer version of "Money" using Exact Audio Copy, a freeware program that is the best of all the PC-based programs I have tried. SACDs can't be read by computers, so to get a version of the DSD data that could be read by Cool Edit Pro, I digitized the analog outputs of a Musical Fidelity Tri-Vista SACD player with a Metric Halo Mobile IO A/D converter connected to my Apple Titanium PowerBook with a FireWire link. To make sure I captured all the audio content of the DSD-encoded layer, I ran the MIO at 96kHz sampling and 24-bit bit depth, and I used Bias Peak 3.2 to create an uncompressed two-channel AIFF file that I ported to my PC for analysis.

To avoid inadvertently clipping the music in the transfer, I used the MIO's excellent meters to keep the peak level below -3dBFS, then normalized the file to 0dBFS using Cool Edit Pro.

The table below shows the statistical data generated by Cool Edit Pro for the two PCM files. Both peak at 0dBFS, though the right channel is loudest for the CD data, the left for the data derived from the DSD layer. There are no clipped samples in the DSD data, but a whopping 362 in the CD data. There is also some DC offset apparent in the latter. More importantly, while the peaks of each file are the same, the average RMS power of the CD data–-15.29dB average of both channels—is significantly higher than that of the SACD layer (17.7dB). No wonder the CD layer sounded louder. This suggests compression or peak limiting was used in the mastering to reduce the song's dynamic range compared with the SACD.

Table 1

Fig.1 shows an FFT-derived spectral analysis of the SACD data, looking at the entire file. A similar spectral analysis for the CD data was more or less identical below 20kHz (vertical white line), which surprised me. Above 20kHz, the musical content on the SACD falls off rapidly, so that above 30kHz or so, all that remains is random noise, presumably from the original analog recorder used to master the recording. (Although SACD has a nominal signal passband to 100kHz, this graph shows that it was perfectly appropriate for me to digitize the DSD-encoded "Money" at 96kHz.) Above 30kHz in this graph, you can see the rising noisefloor typical of DSD data. This results from the aggressive noiseshaping used to get sufficient dynamic range in the audio band from a 1-bit quantizer.

Fig.2 shows the entire "Money" file (SACD version) with time along the x-axis (horizontal), frequency along the y-axis (vertical), and amplitude encoded as color. (White is highest in level, then yellow, red, through purple, to black meaning no content.) The ultrasonic DSD noise can be seen as the pinky/purple hash runnng along the top of the two graphs. (The left channel is on the top, right on the bottom.) The horizontal line at 22kHz indicates the theoretical limit of the CD medium; all that lies above it as far as music in concerned are thin vertical red lines representing Nick Mason's cymbals.

Fig.3 is a more conventional way of looking at the same file data, whereby the green trace is a representation of the analog waveform. The dynamics of the song can be clearly seen. The low-level jingling cash intro swells a little when the familiar riff joins in, and is followed by two verses and the saxophone solo, all of which peak at or below -3dBFS. The first electric guitar solo peaks a little higher, at -1dBFS, and is followed by a dramatic drop in level as the guitar, bass, and electric piano vamp over sparse drum figures. The second guitar solo raises the occasional peak to 0dBFS, with then a drop in intensity for the third and final verse and the fade-out.

Fig.4 shows the waveform display for the CD transfer of "Money." It is very different. The first two verses and the sax solo have had their overall levels raised to be almost as high as the following guitar solo. The solo, however, has the squared-off shape that results when the music has been run through a peak limiter. This is a device that literally chops off the transient peaks, allowing the average level to be higher, hence louder. The vamp in the bridge is now significantly higher and the second guitar solo has the same squared-off shape as the first, with severely clipped transients.

The right channel in the second solo bangs up hard against 0dBFS. This is where the 362 clipped samples reported by Cool Edit Pro come from. Fig.5 therefore shows a very short example from this solo. Yes, the transients in the right channel are flat-topped, suggesting digital clipping. But so are the transients in the left channel, and as this occurs about 0.5dB below 0dBFS, it can't be clipping in the digital domain. It looks as if the mastering engineer who did the transfer for the CD layer ran the analog tape machine into an aggressive peak limiter. He didn't do so for the SACD layer, however, as there is no evidence of flat-topping of the waveform in the SACD version of "Money."

Click on image for larger picture.

Fig.5 "Money," CD layer, waveform during second guitar solo at 4:46.

The ear is relatively forgiving of this type of clipping when it happens on an occasional basis. It is over almost as fast as it happens. But when it occurs over and over again, as it does here, the result is fatiguing. It is, after all, distortion.

Why would someone do this? The cynic would suggest that it was to make the SACD transfer more transparent-sounding than the CD transfer, with less grain and greater dynamic contrasts apparent. Maybe. However, you then have to deal with the fact that to untrained ears, "louder" is always "better," and the CD layer does indeed sound louder.

I suspect, therefore, that the work was done on the CD layer so that it didn't sound too different from the dynamically crippled norm that non-audiophiles have come to expect as "CD sound." Certainly if you do a Google search for comments on this SACD, many people have written how much they like the CD layer, describing it as having more "impact," more "punch."

But the difference negates any comparisons between the two media, at least using this recording.

Jon Iverson Adds Some Comments

I was puzzled when reviewing the DSotM disc: the CD layer sounded more aggressive than the hybrid's SACD tracks. Not having access to the test equipment JA has on hand, I chalked the differences up to varying characteristics of the two analog-to-digital converters (one PCM-based, the other DSD) used for each layer and the more laid-back qualities of SACD sound. In the end, I found myself more closely comparing the SACD layer to the excellent new vinyl release that was mastered at AcousTech and pressed with metal parts by RTI.

Now, with the evidence from JA's graphs, I wonder what the point of limiting the CD's dynamics on a special release like this could possibly be. I've speculated that EMI may have wanted to give the CD layer more "punch" since it is likely the one to be played on the radio. Or perhaps, as JA notes, EMI and Sony have conspired to place DSD in a more audiophile light with this manipulation—which is troubling when you start to ponder which other hybrids might have been altered in this manner.

But, like JA, I'll guess the answer is actually more of a mundane "business-as-usual" attitude at the CD mastering house. The paranoid audiophile in me suspects that the major labels now make it standard practice to push the audio level on all of their rock CDs to give them a more in-your-face sound. The evidence I've read in Mix magazine and that JA and others have gathered would support this contention. I can almost see the young EMI exec jabbing his finger at the mastering engineer and shouting, "The audiophiles have got their prissy SACD layer, now make the other one ROCK!"

So, when it came to the new DSotM re-release, they simply applied the standard sub-standard treatment.